Nanoparticles from supercritical fluid antisolvent process using particle growth and agglomeration retardants

ABSTRACT

The present invention provides a method of forming particles using supercritical fluid (SCF). In accordance with the method, one or more growth retardant compounds having both SCF-philic and SCF-phobic groups are present when one or more solute materials reach a supersaturation point and begin to form particle nuclei. The growth retardant compounds can reduce the particle growth rate, increase the nucleation rate and also prevent particle agglomeration. Preferred growth retardant compounds include sugar acetates and fluorocarbons.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to methods of forming solid particlesusing supercritical fluids. More particularly, the present inventionrelates to methods of forming solid nanoparticles using supercriticalfluids and particle growth and agglomeration retardants.

2. Description of Related Art

Supercritical fluids (SCF) are gases or liquids that are compressedabove their critical pressure and heated above their criticaltemperature. The most commonly employed SCF is carbon dioxide (CO₂)because of its low cost, non-toxicity and ease of availability. Thecritical temperature of CO₂ is 304.2K and the critical pressure of CO₂is 7.38 MPa.

Several properties make SCF ideal for the production of particles. Forexample, commonly used organic solvents have large volumetric expansioncoefficients and large diffusion coefficients in SCF. SCF exhibits arelatively low viscosity when compared to sub-critical liquids. And, SCFcan provide economical operation costs and environmentally benignprocessing.

In recent years, supercritical fluids such as CO₂ have been successfullyused to precipitate particles for various drug delivery systems. The twomost commonly used SCF based particle-processing techniques include theRapid Expansion of Supercritical fluid Solution (RESS) process and theSupercritical fluids Anti-Solvent (SAS) process.

The RESS process involves the expansion of a solution comprising amaterial (e.g., a drug or a drug/polymer mixture) dissolved in SCFthrough a fine nozzle into a low-pressure chamber causing highsupersaturation, nucleation and precipitation of the material dissolvedin the SCF in the form of fine particles. Unfortunately the applicationof this process is limited due to the low polarizability andnon-existent dipole moment of CO₂, which makes it a poor solvent formost pharmaceuticals and biopolymers. Another major disadvantage of theRESS process is that the fine particles produced upon expansion of theSCF solution tend to exhibit a high degree of agglomeration. In somecases, long chains of connected particles are formed, which render theparticles unsuitable for various pharmaceutical applications.

The SAS process uses SCF as an antisolvent to produce particles. Ittakes advantage of the high solubility or miscibility of organicsolvents in SCF. In the SAS process, a solution comprising a material(e.g., a drug or a drug/polymer, lipid and/or wax mixture) dissolved inan organic solvent is injected into the SCF using a fine nozzle. The SCFextracts the organic solvent from the solution thereby causingsupersaturation and nucleation of the material, which precipitates inthe form of fine particles.

Several variations of the SAS process have been developed in order toobtain better control over the size and morphology of the resultingparticles. These variations of the SAS process are known in the art as:Precipitation with Compressed Anti-solvents (PCA) (see, e.g., Schmitt,U.S. Pat. No. 5,707,634); Aerosol Spray Extraction System (ASES) (see,e.g., Fischer et al., U.S. Pat. No. 5,043,280, Lim et al., EP 0 542 314,and Manning et al., U.S. Pat. No. 5,770,559); and the Solvent EnhancedDispersion with Supercritical fluid (SEDS) (see, e.g., Hanna et al.,WO95/01221, WO96/00610 and WO99/59710). The disclosures of all of thepreceding references are hereby incorporated by reference in theirentirety.

Although the techniques referenced above provide several advantages overthe conventional SAS process, they are also subject to certainlimitations. In most cases, the processes mentioned above are unsuitablefor producing particles having diameters smaller than about 500 nm and anarrow size distribution. This is primarily due to the fundamentallimitations imposed by the nucleation and growth phenomenon that occursin the SCF antisolvent process, which tends to precipitate the particlesof most materials in the micron size range. In other words,supersaturation is not reached rapidly enough in most cases to causehigh nucleation rates and precipitation of particles in the nanometerrange.

Different techniques have been developed to increase the supersaturationrate through enhanced mixing of the organic solvent, and the SCF. Thishas usually been achieved using finer diameter nozzles, coaxial nozzles,mixers and ultrasound devices. Although these techniques are effective,they are not universal and are only effective for some materials.Furthermore, these enhanced mixing techniques pose problems forlarge-scale pharmaceutical manufacture and processing. For example theuse of fine nozzles or coaxial nozzles often introduces problems of lowyields and nozzle clogging. Mixers and ultrasound in the supercriticalfluid phase also causes problems of material degradation due to highshear and foreign material contamination in the final product.

Another disadvantage of the conventional and the modified SAS techniqueis that the particles obtained usually tend to form agglomerates. Insome cases, the particles obtained are in the form of a fine mesh ofconnected particles. This is often unsuitable in the pharmaceuticalindustry, especially for the production of particles for respirationwhere agglomeration of particles drastically deteriorates theaerodynamic properties of the final product.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method of forming particles using SCFthat overcomes the limitations of conventional SCF particle processingtechniques. Particles produced in accordance with the present inventionundergo minimal agglomeration and thus possess superior properties ascompared to particles produced in conventional SCF processingtechniques.

The method of the invention employs one or more compounds that aresoluble in SCF and act as particle growth and agglomeration inhibitorsduring precipitation. In all SCF processes, whether SCF is used as asolvent or an antisolvent, particles are formed via supersaturation,which in turn leads to the formation of nuclei. Particles are obtainedwhen the nuclei formed coalesce together to form large stable entities.In the method of the present invention, one or more SCF solublecompounds hereinafter referred to as “growth retardant compounds” areemployed to hinder nuclei coalescence, which results in theprecipitation of smaller particles. Apart from affecting nucleicoalescence, the growth retardants also hinder inter particleinteraction and thus minimize particle agglomeration.

In the preferred embodiment of the invention, CO₂ is used as the SCF,and the growth retardant compounds employed include both CO₂-philic andCO₂-phobic or solute-philic groups. The solubility of the growthretardant compounds in supercritical CO₂ is attributed to the presenceof the CO₂-philic group(s). The interaction between the growth retardantcompounds and the nuclei formed during precipitation, which results in ashielding effect, is attributed to the CO₂-phobic group(s). Examples ofgrowth retardant compounds suitable for use in accordance with themethod of the invention include sugar acetates, fluorocarbons and blockcopolymers comprised of polymer blocks selected from the groupconsisting of polypropylene oxide, polyethylene oxide, poly methacrylicacid (PMMA), poly acrylic acid (PAA), poly vinyl acetate (PVA) andpolyethylene oxide (PEO).

The growth retardant compounds can be either introduced into the SCF aspart of a solution comprising one or more, materials to be precipitated(e.g., a drug or a drug/polymer) dissolved in an organic solvent, or thegrowth retardant compounds can be dissolved in the SCF before thematerial to be precipitated is introduced into the SCF. Both in the RESSand SAS methods of particle processing, during the precipitation stepthe final particle size is determined by the degree of particle growthdue to the nuclei growth and coalescence. The growth retardant compoundspresent in the SCF, or in some cases co-precipitated in the SCF, protector shield the nuclei formed and thereby prevent the particles fromagglomerating into larger particles. Thus, the method of the presentinvention facilitates the manufacture of particles using equipment usedin the conventional SAS and the RESS techniques, but employs one or moregrowth retardant compounds to allow for the precipitation of particleshaving smaller sizes with lesser agglomeration.

The foregoing and other features of the invention are hereinafter morefully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an exemplary apparatus forproducing particles in accordance with a first embodiment of theinvention.

FIG. 2 is a schematic representation of an exemplary apparatus forproducing particles in accordance with a second embodiment of theinvention.

FIG. 3 is a schematic representation of an exemplary apparatus forproducing particles in accordance with a third embodiment of theinvention.

FIGS. 4A and 4B are scanning electron micrographs of particles formed inthe Example below.

FIG. 5 is a graph showing the size distribution of particles formed inthe Example below.

DETAILED DESCRIPTION OF THE INVENTION

As previously noted, in the conventional SAS technique, an organicsolution containing a known amount of a material to be precipitated(e.g., a drug or a mixture of a drug and a polymer, lipid and/or a wax)is injected into a high-pressure vessel containing SCF. Mass transferbetween the organic solvent phase and the supercritical fluid phasecauses supersaturation, which leads to the formation of fine nuclei. Thenuclei grow due to an influx of solute molecules to the surface to formlarger stable particles. The method of the present invention providescontrol over particle size by affecting both nucleation growth kineticsand by preventing agglomeration, which leads to smaller and more uniformparticles.

In one preferred embodiment of the method of the invention, a solutioncomprising one or more materials to be precipitated into particlesdissolved in one or more organic solvents is introduced into a SCF intowhich has been dissolved one or more growth retardant compounds. As soonas the solution is introduced into the SCF, mass transfer between theorganic solvent present in the solution and the SCF phase occurs, whichleads to supersaturation and the formation of nuclei comprising thematerial(s) to be precipitated. The growth retardant compounds presentin the SCF surround the nuclei immediately after they are formed andthereby prevent the nuclei from further rapid growth and agglomeration.Hence, particles are precipitated having sizes much smaller than can beobtained using the conventional SAS technique. It will be appreciatedthat the growth retardant compound(s) can be introduced into the SCFwith the solution as opposed to being dissolved in the SCF prior to theintroduction of the solution into the SCF.

In another preferred embodiment of the method of the invention, one ormore growth retardant compounds are dissolved in the SCF together withone or more materials to be precipitated into particles to form a SCFsolution. The SCF solution is then expanded across a pressure drop,preferably through a nozzle into a low-pressure collection chamber. Therapid decompression of the SCF solution causes supersaturation andnucleation leading to particle precipitation. As observed in the SASprocess, the size of the resulting particles in the RESS process is alsodetermined by the rate at which supersaturation is reached and bynucleation growth kinetics. In the method of the present invention,during expansion of the SCF the growth retardant compound(s) are alsoprecipitated. The growth retardant compound(s) reduce the nuclei growthrate and coalescence of nuclei into larger agglomerates. Hence,particles are precipitated having sizes much smaller than can beobtained using the conventional RESS technique.

The agglomeration of particles, which is a problem in the conventionalSAS and the RESS techniques, is also minimized due to the shieldingeffect provided by the growth retardant compound(s). In some cases, theparticles produced in accordance with the method of the inventionexhibit unusual surface properties or crystalline shape changes. This isdue to the alteration of nuclei growth rates caused by the presence ofthe growth retardant compound(s) during precipitation. The growthretardant compound(s) can hinder the growth of one crystalline facethereby creating particles of differing shape and surfacecharacteristics. After the precipitation process is over, the growthretardant compound(s) can easily be removed and separated from theparticles because the growth retardant compound(s) are soluble in SCF.The growth retardant compound(s) can conveniently be removed simply bypurging the high-pressure precipitation vessel with a SCF such as pureCO₂.

Any compounds that comprise both SCF-philic groups, which make thecompound soluble in SCF, and SCF-phobic groups, which have an affinityor attraction to the nuclei of the material(s) formed during theprecipitation step, can be employed as growth retardant compounds.Examples of growth retardant compounds for use with supercritical carbondioxide (SC—CO₂) include sugar acetates such as sucrose octaacetate andalpha D glucose penta acetate, fluorocarbons such as perfluoropolyethylene(s), and block copolymers comprised of polymer blocksselected from the group consisting of polypropylene oxide, polyethyleneoxide, poly methacrylic acid (PMMA), poly acrylic acid (PAA), poly vinylacetate (PVA) and polyethylene oxide (PEO). When the material(s) to beprecipitated into particles is intended for pharmaceutical applications,the growth retardant compounds should be non-toxic.

Particles produced in accordance with the methods of the invention cancomprise a single-material or a combination of more than one material.For example, it is possible to produce particles of a drug or a drugthat is encapsulated within a coating material or dispersed within amatrix comprising another material such as a polymer, lipid and/or wax.The particles formed in accordance with the methods of the inventiontend to be smaller and/or less agglomerated than particles obtainedusing conventional SCF processing techniques including SAS, RESS, PCA,ASES and SEDS.

A schematic representation of an exemplary apparatus for producingparticles in accordance with a first embodiment of the invention isshown in FIG. 1. The apparatus 10 includes a vessel 20, which ispreferably tubular and has an inner cylindrical sidewall and first andsecond ends that are spaced apart from each other to define acylindrical chamber 30. A supercritical fluid pump 40 and a solutionfeed pump 50 communicate with the chamber 30. A release valve 60 and abackpressure regulator 70 also communicate with the chamber 30. Athermostat (not shown) controls heating elements 80 that are locatedproximate to the vessel 20. Disposed within the chamber 30 are asolution inlet nozzle 90, a supercritical fluid inlet 100, and a filter(not shown) to collect the particles.

For the laboratory-scale production of particles, the supercriticalfluid pump is preferably a P-200 high-pressure reciprocating pumpcommercially available from Thar Technologies, Inc. (Pittsburgh, Pa.).Suitable alternative pumps include diaphragm pumps and air-actuatedpumps that provide a continuous flow of supercritical fluid. Thesupercritical fluid pump 40 is in fluid communication with thesupercritical fluid inlet 100, and thereby supplies supercritical fluidinto the chamber 30.

For the laboratory-scale production of particles, the solution feed pump50 is preferably a high-pressure liquid chromatography (HPLC)reciprocating pump such as the model PU-2080, which is commerciallyavailable from Jasco Inc. (Easton, Md.). Suitable alternative pumpsinclude other reciprocating pumps, diaphragm pumps and syringe typepumps, such as the 1000D or 260D pumps, which are commercially availablefrom Isco Inc. (Lincoln, Nebr.). The solution feed pump 50 is in fluidcommunication with the solution inlet nozzle 90, and thereby suppliessolution into the chamber 30. The solution inlet nozzle 90 is preferablya capillary-type tube, or a tube having non-circular cross-section, forexample, a slit, and preferably extends through the sidewall into thechamber 30.

For the laboratory-scale production of particles, the backpressureregulator is preferably a 26-1700 type regulator, which is commerciallyavailable from Tescom, USA (Elk River, Minn.) and is interchangeablewith other like valves that are known to those of ordinary skill in theart. If desired, a mixer assembly 110 that includes a motor, a shaftextending from the motor and a rotor disposed at a distal end of theshaft, can be located in the chamber 30 in order to ensure adequatemixing.

Preferably, a controller (not shown) communicates with and controls thesupercritical fluid pump 40, the solution feed pump 50, the releasevalve 60, the backpressure regulator 70, the thermostat for the heatingelements 80, and the mixer assembly 110. Suitable controllers are wellknown in the art and are interchangeable therewith.

A solution 120 is pumped by the solution feed pump 50 to the chamber 30.The solution 120 comprises one or more solvents, one or more materialsto be precipitated in the form of particles (sometimes hereinafterreferred to as “solute(s)”), and one ore more growth retardantcompounds. The solvent must be at least partially soluble insupercritical fluid. Preferred solvents include alcohols, dimethylsulfoxide (DMSO), dimethyl formamide (DMF), tetra hydrofuran (THF),acetone, ethyl acetate and methylene chloride.

The material or materials that are to be precipitated into particles(i.e., the solute(s)) can comprise any material that is soluble in thesolvent(s). Because the particles produced in accordance with the methodof the invention tend to be very small in size and less agglomerated,the material to be precipitated into particles is often apharmaceutical. Suitable pharmaceutical solute materials include, forexample, medicinal agents, biologically active materials, sugars, viralmaterials, diagnostic aids, nutritional materials, proteins, peptidesand animal and/or plant extracts. The material can also comprise one ormore non-pharmaceutical solute materials such as, for example,agricultural chemicals, dyes, explosives, paints, polymer precursors,alkyloids, alkaloids, cosmetics, insecticides, pigments, toxins,antigens, enzymes, catalysts, nucleic acids, and combinations thereof.

It will be appreciated that the method of the invention can be utilizedto produce particles comprising two or more different solute materials.If multiple soluble materials are dissolved in the solvent, theresultant particles will tend to contain all of the solute constituents.If micro-encapsulates, microspheres, coated particles or co-precipitatedparticles are desired, a carrier or matrix material can be dissolved inthe same solution with a drug or other solute material. Preferred matrixmaterials include polymers, fillers, disintegrants, binders,solubilizers, excipients, and combinations thereof. In particular, thematrix materials can be, for example, polysaccharides, polyesters,polyethers, polyanhydrides, polyglycolides (PLGA), polylactic acids(PLA), polycaprolactones (PCL), polyethylene glycols (PEG), andpolypeptides.

In the presently most preferred embodiment of the invention, the SCF isSC—CO₂. Carbon dioxide is supercritical when certain environmentalparameters are met, namely, when the carbon dioxide is above about 304.2Kelvin (K) and above about 7.38 megaPascal (MPa). Suitable alternativesupercritical fluids include nitrous oxide, dimethylether, straightchain or branched C1-C6-alkanes, alkenes and combinations thereof.Preferable alkanes and alcohols include ethane, propane, butane,isopropane and the like. The SCF chosen must be compatible with thesolute material(s) to be used in the process. It is important that thesolute material(s) be generally insoluble in the SCF, whereas thesolvent(s) should be generally soluble in the SCF.

The growth retardant compound can be any compound that is soluble in theSCF and possesses both SCF-philic and either SCF-phobic or solute-philicgroups. The growth retardant compounds (e.g., sugar acetates containingcarbonyl groups) can undergo Lewis acid and Lewis base interactions withthe SCF, thereby readily dissolving in them and causing a growthretardant effect upon co-precipitation due to interaction with thesolute. Preferred growth retardant compounds include, for example, sugaracetates, fluorocarbons and block copolymers comprised of polymer blocksselected from the group consisting of polypropylene oxide, polyethyleneoxide, poly methacrylic acid (PMMA), poly acrylic acid (PAA), poly vinylacetate (PVA) and polyethylene oxide (PEO).

The first embodiment of the method of the invention will be describedwith reference to FIG. 1. The apparatus 10 is first assembled. Thesupercritical fluid pump 40 is used to supply supercritical fluid intothe chamber 30 up to a predetermined pressure at a constant flow rate.The pressure inside the vessel 20 is maintained constant using thebackpressure regulator 70. A thermostat that controls the heatingelements 80 is used to maintain the temperature of the vessel 20 at apredetermined temperature. Once the vessel 20 is pressurized with SCF atthe desired operating temperature, pressure and flow rate, the solutionfeed pump 50 is used to supply the solution 120 comprising a solvent, asolute material to be precipitated and the growth retardant compoundthrough the solution inlet nozzle 90 and into the chamber 30.

If a mixing assembly 110 is employed within the chamber 20, the mixingassembly 110 is engaged so that the motor rotates the rotor prior to theintroduction of the solution 120 into the SCF-filled chamber 20. Thespinning rotor is employed to intimately mix SCF with the solutionduring the precipitation process.

As soon as the liquid solution is introduced into the vessel, masstransfer of the solvent present in the solution into the SCF results insupersaturation and nucleation of the solute material. The growthretardant compound present in the solution also immediately dissolves inthe supercritical fluid phase. In convention processes, the nuclei ofsolute material obtained in the vessel now undergo growth andcoalescence and thus form larger stable particles. In the method of thepresent invention, however, the growth retardant compound present in theSCF reduces the growth rate of the nuclei and thereby increases thesolution supersaturation and minimizes the opportunities the nuclei haveto coalesce and form agglomerates. The particles precipitated in thechamber 30 can be collected from the bottom of the chamber. A mixturecomprising the SCF, the solvent(s) and the growth retardant compound(s)is removed from the chamber through the backpressure regulator 70 andthe filter. The filter separates the particles present in the vesselfrom the SCF stream.

After the particles have been precipitated, the particles are typicallysubjected to a cleaning step whereby any residual solvent and/or growthretardant compound present inside the chamber is removed. The flow ofsolution 120 into the vessel 20 is stopped. However, the flow of SCFthrough the vessel 20 is maintained for a time sufficient to purge theresidual solvent and the growth retardant compound present in thesupercritical fluid phase inside the vessel. After cleaning, the vesselis depressurized to obtain the particles.

The resultant particles can include crystalline, semi-crystalline andamorphous powders of small-molecules, powders of polymeric andbiological molecules, specifically but not limited tobiologically-active medicinal substances, therapeutic proteins andpeptides intended for different drug delivery applications. Theparticles can be in the form of spheres or capsules, and can include,for example, a combination of therapeutic or biologically active agentscoated or incorporated into a carrier polymer or excipient. The spheresand capsules are generally suitable for controlled, sustained ormodified drug release, taste masking or modifying, and drugsolubilization. Particles produced in accordance with the methods of theinvention preferably have an average particle size of less than 10micron and more than 300 nm.

A schematic representation of an exemplary apparatus 11 for producingparticles in accordance with a second embodiment of the invention isshown in FIG. 2. Because the apparatus 11 shown in FIG. 2 is similar, inmany respects, to the apparatus 10 shown in FIG. 1, the same referencenumbers used in FIG. 1 are also used to identify identical components ofthe apparatus 11 shown in FIG. 2. The primary difference between theapparatus 10 shown in FIG. 1 and the apparatus 11 shown in FIG. 2 is theaddition of a high-pressure extraction column 130 between the SCF pump40 and the SCF inlet 100.

In accordance with the second embodiment of the invention, a thermostatcontrols heaters that are used to maintain the temperature of theextraction column 130 at a predetermined constant temperature. Theextraction column 130 is packed with one or more suitable growthretardant compounds. SCF from the SCF pump 40 flows through theextraction column 130 and becomes saturated with the growth retardantcompound(s) prior to entering the chamber 30. Thus unlike the firstembodiment of the invention where the growth retardant compound wasintroduced with the solution 120 through the solution inlet nozzle 90,in the second embodiment of the invention the growth retardant compoundis pre-dissolved in the SCF prior to the introduction of the SCF in thechamber 30.

Although the growth retardant compound is introduced in a differentmanner, particle precipitation occurs in the second embodiment of theinvention in essentially the same manner as in the first embodiment. Asolution 121, which comprises one or more solvents and one or moresolute materials to be precipitated, is introduced into the chamber 30containing the SCF and the pre-dissolved growth retardant compound. Masstransfer of the solvent(s) present in the solution into the SCF resultsin supersaturation and nucleation of the solute material(s). As in thefirst embodiment of the invention, the growth retardant compound shieldsthe nuclei of solute material(s) by surrounding the surfaces thereof,which limits or minimizes the opportunity for the nuclei to coalesce andform large particles. The growth retardant compound also prevents interparticle interaction once precipitation is complete, and thus aids inpreventing particle agglomeration.

The particles can be collected from the bottom of the vessel 20. Themixture of SCF, solvent and growth retardant compound is removed fromthe chamber through the backpressure regulator 70 and a filter (notshown). Use of a filter helps separate any particles that may be presentin SCF stream.

Once the precipitation process is complete the flow of solution 121 intothe vessel 20 is stopped. Pure SCF is introduced into the precipitationvessel 20 by bypassing the extraction column 130 containing the growthretardant compound(s) using valves. The flow of pure SCF through thevessel 20 is maintained for a time sufficient to completely purge anyresidual solvent and the growth retardant compound present in thesupercritical fluid phase inside the vessel. After cleaning, the vesselis depressurized to obtain the particles of the solute material(s).

A schematic representation of an exemplary apparatus 12 for producingparticles in accordance with a third embodiment of the invention isshown in FIG. 3. Because the apparatus 12 shown in FIG. 3 is similar, inmany respects, to the apparatuses 10, 11 shown in FIGS. 1 and 2, thesame reference numbers used in FIGS. 1 and 2 are also used to identifyidentical components of the apparatus 12 shown in FIG. 3. The primarydifference between the apparatus 11 shown in FIG. 2 and the apparatus 12shown in FIG. 3 is that there is no solution feed pump or solution inletnozzle to direct solution into the chamber 30 of the vessel 20. In thethird embodiment of the invention, the growth retardant compound(s) andthe solute material(s) to be precipitated are dissolved in the SCF andintroduced in the chamber 30 with the SCF through a fine nozzle 140.

In the third embodiment of the invention, one or more suitable growthretardant compounds and one or more solute materials to be precipitatedare packed into the extraction column 130. A thermostat controls heatersthat are used to maintain the temperature of the extraction column 130at a predetermined constant temperature. SCF from the SCF pump 40 flowsthrough the extraction column 130 and gets saturated with the growthretardant compound(s) and the solute material(s) to be precipitatedprior to entering the chamber 30 of the vessel 20. The third embodimentof the invention is different than the first embodiment, where thegrowth retardant compound(s) and the solute material(s) to beprecipitated are introduced into the chamber 30 as part of a solution120 through the solution inlet nozzle 90. The third embodiment of theinvention is different than the second embodiment, where the solutematerial(s) to be precipitated are introduced into the chamber 30 aspart of a solution 121 through the solution inlet nozzle 90. Instead, inthe third embodiment of the invention, both the solute material(s) to beprecipitated and the growth retardant compound(s) are pre-dissolved inthe SCF and are co-injected into the chamber through the SCF nozzle 140.

Particle precipitation occurs in the third embodiment of the inventiondue to rapid expansion of the SCF in the chamber 30 of the vessel 20.Expansion of the SCF diminishes its solvent power resulting insupersaturation and nucleation of the solute material(s) dissolved init. Not unlike the previously discussed embodiments of the invention,the final particle size of the particles formed in the third embodimentof the invention is also determined by nucleation kinetics. The growthretardant compound(s) present during expansion of the SCF shield thenuclei of solute material(s) by surrounding their surface, therebyminimizing the opportunities for the nuclei to coalesce and form largeparticles. The growth retardant compound(s) prevent inter particleinteraction during and after precipitation, which aids in the preventionof particle agglomeration.

During operation of the apparatus 12 shown in FIG. 3, thermostats areused to control the temperature of the extraction column 130 and thevessel 20. The SCF pump 40 supplies SCF into the extraction column 130,Which contains the growth retardant compound(s) and the solutematerial(s) to be precipitated into particles thereby forming asaturated SCF solution. The saturated SCF solution is then expandedacross a pressure drop into the chamber 30 of the vessel 20, preferablythrough the SCF nozzle 140. The pressure inside the precipitationchamber is maintained well below the critical pressure of the SCF, andpreferably at atmospheric pressure, in order to facilitate maximumexpansion of the SCF solution. If necessary, the backpressure regulator,70 can be used to adjust the pressure inside the chamber 30. Once theprecipitation process has been completed, the flow of the saturated SCFsolution into the chamber 30 is stopped and the particles can berecovered from the chamber 30.

If desired, pure SCF can be introduced into the chamber 30 by bypassingthe extraction column 130 containing the growth retardant compound(s)and the solute material(s) to be precipitated using valves. The pressureand temperature of the pure SCF stream and the pressure and temperaturewithin the chamber 30 is maintained such that only residual growthretardant compound(s) present in the chamber 30 will become dissolved inthe SCF. The flow of pure SCF through the vessel is maintained for atime sufficient to completely purge the growth retardant compoundpresent inside the chamber 30.

The following example is intended only to illustrate the invention andshould not be construed as imposing limitations upon the claims. Unlessspecified otherwise, all materials and equipment used in the examplescan be obtained from Sigma Aldrich, Inc. (St. Louis, Mo.) and/or FisherScientific International, Inc. (Hanover Park, Ill.).

EXAMPLE

Acetaminophen particles were precipitated using the apparatus for thelaboratory-scale production of particles described in the firstembodiment of the invention. Specifically, 3.0 g of acetaminophen (thesolute to be precipitated) and 3.0 g of alpha D glucose penta acetate(the growth retardant compound) were dissolved in 60 g of acetone (thesolvent) to form a clear solution. Supercritical carbon dioxide(“SC—CO₂”) was used as the supercritical fluid. The flow rate of SC—CO₂was set at 75 g/min and the flow rate of the solution was set at 1.5ml/min. A 150-micron nozzle was used to introduce the solution into theSC—CO₂. The operating temperature was set to 40° C. and the operatingpressure was set to 80 bar. As soon as the solution was introduced, masstransfer of acetone into the SC—CO₂ occurred, which led tosupersaturation and the formation of nuclei of acetaminophen in thepresence of alpha D glucose penta acetate. The alpha D glucose pentaacetate acted as a growth retardant compound in that it immediatelysurrounded the acetaminophen nuclei, thereby preventing them fromcoalescing to form larger particles.

After precipitation was complete, the alpha D glucose penta acetatepresent in the precipitation vessel was removed using a continuous flowof pure SC—CO₂ at 40° C. and 80 bar. The vessel was then depressurizedand the particles were collected for analysis.

FIGS. 4A and 4B are scanning electron micrographs of the acetaminophenparticles formed in the Example at two magnifications. The micrographsshow that the acetaminophen appear to be substantially uniform in size(−1-2 microns) and circular in shape. Some minimal particleagglomeration or bridging is apparent.

The mean diameter of the acetaminophen particles formed in the Examplewas measured using the conventional laser light scattering technique,which is known in the art. FIG. 5 is a graph showing the sizedistribution of particles. The mean volume diameter of the particles was4.08 microns and the standard deviation was 1.87 microns.

The foregoing Example demonstrates that particles precipitated in thepresence of a growth retardant compound tend to have a smaller particlesize and a greater degree of uniformity (both in size and in morphology)than can be obtained using conventional supercritical fluid anti-solventtechniques.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and illustrative examples shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general inventive concept asdefined by the appended claims and their equivalents.

1. A method of producing particles using supercritical fluid (SCF)comprising: providing a source of SCF; providing a solution comprising:at least one solvent that is at least partially soluble in the SCF; atleast one solute material that is at least partially soluble in thesolvent and substantially insoluble in the SCF; and at least one growthretardant compound that is at least partially soluble in the SCF andincludes at least one functional group or portion that is SCF-philic andat least one functional group or portion that is SCF-phobic or solutematerial-philic; and contacting the solution and the SCF together underconditions whereby the solvent diffuses into the SCF causingsupersaturation and nucleation of particles comprising the solutematerial, said particles having a smaller size and a reduced amount ofagglomeration than if no growth retardant compound was present.
 2. Themethod according to claim 1 wherein the SCF is supercritical carbondioxide.
 3. The method according to claim 2 wherein the growth retardantcompound is selected from the group consisting of sugar acetates,fluorocarbons and block copolymers.
 4. The method according to claim 2wherein the block copolymer is comprised of polymer blocks selected fromthe group consisting of polypropylene oxide, polyethylene oxide, polymethacrylic acid (PMMA), poly acrylic acid (PAA), poly vinyl acetate(PVA) and polyethylene oxide (PEO).
 5. The method according to claim 1wherein the solute material is selected from the group consisting ofmedicinal agents, biologically active materials, sugars, viralmaterials, diagnostic aids, nutritional materials, proteins, peptides,animal extracts, plant extracts and combinations thereof.
 6. The methodaccording to claim 5 wherein the solution further comprises a secondsolute material selected from the group consisting of polymers, fillers,disintegrants, binders, solubilizers, excipients, and combinationsthereof. In particular, the matrix materials can be, for example,polysaccharides, polyesters, polyethers, polyanhydrides, polyglycolides(PLGA), polylactic acids (PLA), polycaprolactones (PCL), polyethyleneglycols (PEG), polypeptides and combinations thereof.
 7. The methodaccording to claim 6 wherein the particles have an average particle sizeof less than 10 micron and more than 300 nm.
 8. A method of producingparticles using supercritical fluid (SCF) comprising: providing a sourceof SCF; providing a solution comprising: at least one solvent that is atleast partially soluble in the SCF; and at least one solute materialthat is at least partially soluble in the solvent and substantiallyinsoluble in the SCF; dissolving at least one growth retardant compoundin the SCF, the growth retardant compound including at least onefunctional group or portion that is SCF-philic and at least onefunctional group or portion that is SCF-phobic or solutematerial-philic; and contacting the solution and the SCF comprising thedissolved growth retardant compound together under conditions wherebythe solvent diffuses into the SCF causing supersaturation and nucleationof particles comprising the solute material, said particles having asmaller size and a reduced amount of agglomeration than if no growthretardant compound was present.
 9. The method according to claim 8wherein the SCF is supercritical carbon dioxide.
 10. The methodaccording to claim 9 wherein the growth retardant compound is selectedfrom the group consisting of sugar acetates, fluorocarbons and blockcopolymers.
 11. The method according to claim 10 wherein the blockcopolymer is comprised of polymer blocks selected from the groupconsisting of polypropylene oxide, polyethylene oxide, poly methacrylicacid (PMMA), poly acrylic acid (PAA), poly vinyl acetate (PVA) andpolyethylene oxide (PEO).
 12. The method according to claim 8 whereinthe solute material is selected from the group consisting of medicinalagents, biologically active materials, sugars, viral materials,diagnostic aids, nutritional materials, proteins, peptides, animalextracts, plant extracts and combinations thereof.
 13. The methodaccording to claim 12 wherein the solution further comprises a secondsolute material selected from the group consisting of polymers, fillers,disintegrants, binders, solubilizers, excipients, and combinationsthereof. In particular, the matrix materials can be, for example,polysaccharides, polyesters, polyethers, polyanhydrides, polyglycolides(PLGA), polylactic acids (PLA), polycaprolactones (PCL), polyethyleneglycols (PEG), polypeptides and combinations thereof.
 14. The methodaccording to claim 13 wherein the particles have an average particlesize of less than 10 micron and more than 300 nm.
 15. A method ofproducing particles using supercritical fluid (SCF) comprising:providing a source of SCF; dissolving at least one solute material andat least one growth retardant compound in the SCF to form an SCFsolution, wherein the growth retardant compound includes at least onefunctional group or portion that is SCF-philic and at least onefunctional group or portion that is SCF-phobic or solutematerial-philic; and expanding SCF solution across a pressure drop belowthe critical pressure of the SCF whereby the SCF decompresses and causessupersaturation and nucleation of particles comprising the solutematerial, said particles having a smaller size and a reduced amount ofagglomeration than if no growth retardant compound was present.
 16. Themethod according to claim 15 wherein the SCF is supercritical carbondioxide.
 17. The method according to claim 16 wherein the growthretardant compound is selected from the group consisting of sugaracetates, fluorocarbons and block copolymers.
 18. The method accordingto claim 17 wherein the block copolymer is comprised of polymer blocksselected from the group consisting of polypropylene oxide, polyethyleneoxide, poly methacrylic acid (PMMA), poly acrylic acid (PAA), poly vinylacetate (PVA) and polyethylene oxide (PEO).
 19. The method according toclaim 15 wherein the solute material is selected from the groupconsisting of medicinal agents, biologically active materials, sugars,viral materials, diagnostic aids, nutritional materials, proteins,peptides, animal extracts, plant extracts and combinations thereof. 20.The method according to claim 19 wherein the solution further comprisesa second solute material selected from the group consisting of polymers,fillers, disintegrants, binders, solubilizers, excipients, andcombinations thereof. In particular, the matrix materials can be, forexample, polysaccharides, polyesters, polyethers, polyanhydrides,polyglycolides (PLGA), polylactic acids (PLA), polycaprolactones (PCL),polyethylene glycols (PEG), polypeptides and combinations thereof. 21.The method according to claim 20 wherein the particles have an averageparticle size of less than 10 micron and more than 300 nm.